Perilla Mint - Weed of the Week

A Pasture Plant of Concern

Can you think of a plant with medicinal and culinary attributes, yet it’s often toxic to livestock? The weed in question is perilla mint (Perilla frutescens). Perilla mint is becoming more and more common in pastures across West Virginia, causing concern for some.

Sometimes known as beefsteak plant, Chinese basil or purple mint, perilla mint was introduced from East Asia for its ornamental and culinary values. Usually, animals do not consume this plant, but poisoning can occur when more desirable plants are in short supply. After consumption, livestock can develop symptoms related to respiratory distress such as acute inflammation of the lungs, causing fatality in some.

Perilla Mint Identification

Perilla mint is an annual weed prevalent in pastures, periphery of woodlots and occasionally, in gardens and other disturbed fields. It can be easily identified by its ribbed, square stems and broad leaves, arranged oppositely with a toothed margin. The leaves vary in color from green to purple, especially when found growing in the shade, and boast a minty aroma when crushed.

It produces a spindly, hairy spike (raceme) bearing purple or white flowers in pairs. It comes to bloom from July to October and is especially toxic to animals when in bloom. Although not all varieties of perilla mint are considered toxic to animals, and its toxicity can vary due to environmental conditions, this plant must be controlled in pastures or other areas frequented by animals.

Perilla Mint Control

Perilla mint is most susceptible to control during the spring months when the plants are young and actively growing. Once they come to bloom, the plants become hardy and more difficult to control. As an annual, it can be removed mechanically by taking advantage of its shallow root system. Herbicides, such as 2,4-D (low-volatile ester formulations) or tank-mixtures containing both 2,4-D and dicamba (several formulations) along with a surfactant, provide good control when applied in early spring. Herbicides containing aminopyrlid (Milestone, Grazon Next) also provide effective control, but may persist in treated hay and in manure derived from animals that were fed with treated forage or hay.

Perilla Mint and Livestock

Perilla mint continues to be a problem weed in West Virginia pastures. Multiple animal fatalities were attributed to this weed in 2019 and 2018.

In fall 2018, a request was made by a veterinarian in the eastern panhandle region to help identify poisonous weeds in three different pastures where there were 35 cow fatalities. At all three locations, perilla mint was noted in large quantities where animals grazed actively.

Abnormal breathing is manifested in affected animals referred to as atypical interstitial pneumonia syndrome (AIP).  

In 2018, excess moisture in the region caused in suboptimal growing conditions, which resulted in poor forage stand. Whereas in 2019, drought-like conditions in late summer and early fall resulted in similar conditions. Producers involved did not realize that until it was too late.

Such events require producers to monitor pastures for good stands of desirable plant species prior to grazing events and proper management of poisonous weeds at the same time. Other factors, such as high amounts of certain toxic metabolites like tryptophan/3-methylindole present in lush forages, nitrous oxides present especially around silos and allergens from moldy feed, also can cause varying levels of toxicity to animals.

The volatile oils present in perilla mint contain several compounds in varying amounts. Among these, the perilla ketones are considered to me most toxic to animals and are present in high quantities during later stages (August to October) of the growing season, especially in seeds.

Animal feeding studies have revealed that perilla mint in the seed stage was most lethal. Animal fatality occurred within three days when a 174-pound calf was fed with 5 pounds of perilla mint hay in the seed stage along with 5 pounds of regular hay. A 291-pound calf fed for a period of 15 days with similar amounts of perilla mint hay in the green stage collected in July, failed to demonstrate any clinical signs. A third calf weighing 398 pounds fed for three days with 14.5 pounds per day of dry hay collected in January of the previous year also failed to demonstrate any clinical signs.  

Author: Rakesh Chandran, WVU Extension Weed Science Specialist
Last Reviewed: April 2020

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